Setup a new machine in Virtualbox, I gave mine 1.5Gb RAM and 20GB space. Windows 8 requires 1Gb RAM and 13Gb storage, so make sure you're over that.

Set your OS as Windows 7 64-bit.

There are some settings that is required for this to work. In the settings, go to System > Motherboard and enable IO APIC extended settings, then move to the Processor tab enable PAE/NX, then move to the Acceleration tab and enable the 2 hardware acceleration (Nested Paging and VT-x/AMD-V). (Most important part, why I even wrote this post).

Start the machine, select the mounted drive and everything is common sense from there to install the OS.